Matthew is 3 times as old as Jenny. In 7 years, he will be twice as old as she will be then. How old is she now?

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Now: Let Jenny be "x", then Matthew is "3x".

In 7 years, Matthew will be 3x + 7, and Jenny is x + 7.

Matthew is twice as old as Jenny, therefore

`3x + 7 = 2(x + 7)`

`3x + 7 = 2x + 14`

`x + 7 =...

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Now: Let Jenny be "x", then Matthew is "3x".

In 7 years, Matthew will be 3x + 7, and Jenny is x + 7.

Matthew is twice as old as Jenny, therefore

`3x + 7 = 2(x + 7)`

`3x + 7 = 2x + 14`

`x + 7 = 14`

`x = 7`

Jenny is 7 years old now.

Check:

Now:  Jenny is 7, which makes Matthew 21, 3 times as old.

In 7 years, Jenny will be 14 and Matthew will be 28, which is twice as old.

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